The present invention relates in general to range error correction of spinning, gun-launched artillery projectiles and, in particular, to deployment of a drag braking device for such projectiles.
In recent years, various concepts for increasing the accuracy of inventory artillery projectiles have been proposed. One of these, the range-correction concept, assumes that the gun is purposely aimed to overshoot its intended target. Using in-flight measurements, the range to impact of a projectile that is subjected to a host of launch and flight disturbances is estimated early in the trajectory. At a time determined by the range estimate, a drag-inducing device is deployed causing the projectile to impact at the intended target. Generically, this concept is called xe2x80x9cDragster.xe2x80x9d
The present invention is a simple and inexpensive apparatus and method for performing the range-to-impact estimation and commanding drag brake deployment when indicated. The apparatus incorporates a spin sensor, two axial acceleration sensors, and a processor. The drag brake used in the present invention is, for example, a D-ring type as disclosed in U.S. Pat. No. 5,816,531 entitled xe2x80x9cRange Correction Module for a Spin Stabilized Projectilexe2x80x9d issued to M. Hollis and F. Brandon on Oct. 6, 1998, which patent is hereby expressly incorporated by reference. The present invention has been named the D-ring Dragster fuze. In the D-ring Dragster fuze, all the components required to implement range correction can be incorporated as a modification of an existing artillery fuze and still satisfy operational requirements for fuze shape factor.
The metrics for accuracy improvement that make a Dragster system worthwhile are very much application-specific. The ability to deliver artillery fire onto a target is affected by many factors, some of which may not even be functions of the weapon system, e.g., target location error (TLE) and technique of fire. In cases where the impact locations of projectiles relative to their targets can either be observed or otherwise known, the aiming of subsequent rounds is adjusted until the desired impact locations are achieved. This technique is called xe2x80x9cadjusted firexe2x80x9d. At longer ranges where adjusted fire techniques are seldom desirable or practical, a technique called xe2x80x9cpredicted firexe2x80x9d is almost exclusively used. In predicted fire, the most current meteorological (MET) data and weapon system information are used along with a firing algorithm to generate an aiming solution to the location that has been identified as containing targets.
When using predicted fire, conventional uncorrected artillery projectiles have an elliptical fall of shot pattern with the range axis greatly exceeding the deflection axis. The purpose of a Dragster system is to reduce the range errors. However, there is little or no benefit in achieving range dispersions smaller than the associated deflection dispersions considering that there is no correction capability for the deflection errors in the postulated one-dimension correction system. Thus, the operational goal for Dragster is to achieve a fall of shot pattern centered at the aim point with the range errors roughly equal to the dispersion errors.
This goal leads to the requirement that the dispersion of the errors in the range-to-impact estimates be no greater than the deflection dispersion of uncorrected projectiles at that target range. Evaluation of the apparatus and method disclosed herein using a computerized six degree-of-freedom trajectory code shows this requirement is met. Significant reductions in range errors for the simulated Dragster rounds were achieved with the fall of shot patterns estimated for the Dragster rounds all approximately circular. This same basic result of the Dragster fuze achieving range errors roughly equal to dispersion errors would be anticipated for predicted fire of improved systems (e.g., better MET information) that would reduce dispersion of conventional rounds.
Known Dragster concepts either require in-flight information from external sources or actions by the weapon""s crew beyond the current tactical procedures. Dragster systems proposed heretofore have included communication links, global positioning system (GPS) receivers, unique Dragster rounds, and/or a unique Dragster firing technique. The fuze-configured D-ring Dragster disclosed herein is fire-and-forget and makes no additional demands on the weapon crew. The only operational differences are the installation of Dragster fuzes on the projectiles (rather than some other fuze) and the selection of a Dragster mission in the weapon""s fire control computer. These differences represent alternative choices for already required actions.
A Dragster fuze under development by others is known as STAR (Smart Trajectory Artillery Round). STAR differs from the Dragster apparatus disclosed herein in that STAR incorporates a GPS receiver to track the trajectory and provide inputs to the range-to-impact estimator.
The present invention makes a range-to-impact estimate (i.e., ground level distance) by comparing an on-board path length measurement (i.e., at a given time t, the distance the projectile has traveled along its trajectory) to a nominal path length provided by the weapon fire control computer (and loaded into a memory in the fuze prior to launch). This comparison is made early in the trajectory and the estimated overshoot distance is used to determine the time of deployment of the braking device. The on-board path length measurements are made using the outputs of a magnetic field strength sensor and two linear accelerometers. With appropriate processing, the magnetic sensor determines projectile spin rate and the accelerometers determine projectile drag. In turn, the spin rate is used to infer muzzle velocity and the drag is used to update the projectile""s speed. Numerical integration then gives distance.
Kurschner, Erdmann, and Crist disclose the use of a magnetic sensor to calculate spin rate and muzzle velocity of spinning projectiles (See U.S. Pat. No. 5,497,704). Though Dragster estimates the same rates (i.e., projectile spin and muzzle velocity) from turn counts in the earth""s magnetic field as does Kurschner""s device, the Dragster further uses the spin rate estimate in conjunction with calibration data to remove measurement bias from the axial acceleration sensors outputs. Additionally, the method for deriving projectile spin rate from magnetic turn counts differs from that of Kurschner et al. by including processing to compensate for the potential difference between projectile spin rate and magnetic field crossing rate created by projectile yawing motion.